27 April 2000

New Technique To Strip Gene Info From Donor Eggs

by Kate Melville

A combined team of French, Spanish and Italian fertility experts report today (Thursday 27 April) in the journal Human Reproduction* that they have developed a novel method of membrane fusion which would allow the nucleus from the egg of an infertile women to be successfully transferred into the cytoplasm of a donor egg from which the nucleus had been removed. As the nucleus carries most of the genes, transferring it to an enucleated donor egg means that the infertile woman would give birth to a baby who would be almost entirely genetically her own as well as her partner's.

The technique could help those women whose embryos have repeatedly failed to develop because of defects in the extranuclear material of their own eggs (the cytoplasm). Although cytoplasmic defects account for less than ten per cent of women attending IVF clinics, these women can currently only be treated by egg donation.

Cell fusion techniques, used mostly for cloning mammalian eggs, involve electrofusion and are not suitable for human eggs because the eggs are almost always activated by the process.

In the experiments reported today on human eggs, the researchers have successfully transferred nuclei by two methods -- one using a chemical, the other using a deceptively simple mechanical technique.

In the chemical method, the nucleus of one egg was 'glued' to another egg from which the nucleus had been removed using an organic chemical, phytohaemagglutinin. Then the pair of eggs were induced to fuse by treating them with another organic chemical, polyethylene glycol (PEG).

In the mechanical method, the Œ'glued' eggs were induced to fuse by exquisitely delicate micromanipulation with the type of micropipette used in ICSI (the fertility technique whereby a single sperm is introduced into an egg).

Head of the research team Jan Tesarik PhD MD, from Laboratoire d'Eylau in Paris, said: "Both methods yielded a high proportion of fused cells without causing any activation of the eggs. We believe that this is the first time the mechanical technique has ever been tried and we were surprised and encouraged how successful it was. The use of phytohaemagglutin first was a major factor in the success of this method."

Dr Tesarik said that no attempts were made to fertilize the reconstructed eggs because the formation of human embryos for research purposes was banned in France and Spain and strictly regulated in Italy.

"We are now ready to try and develop treatment for women with repeated failures of embryo development caused by defective cytoplasm in their eggs. Compared to standard egg donation this approach will enable a nearly full genetic contribution (with the exception of 37 female extranuclear genes) from both parents to the future embryo.

The researchers believe that their 'clean' method of using the cytoplasm of the donated egg and transferring the patient's nucleus overcomes potential problems that could arise from accidental mixing of important extranuclear genes (mitochondrial DNA) from patient and donor.

"Because ICSI will probably be the best way of fertilising the reconstructed eggs the mechanical method we've developed will have the advantage of simultaneously fusing the eggs and introducing the sperm in a single, relatively simple action," said Dr Tesarik. "We are currently testing the efficiency of this technique in the laboratory but it could be some considerable time before we are ready to offer the service to couples."